Pneumatic rubber tire treads are typically configured with a circumferential outer cap rubber layer which contains a running surface for the tire for contacting the road, and an underlying tread base rubber layer (underlying said outer tread cap rubber layer).
The outer tread cap layer is typically intended to promote a tire running surface with suitable resistance to tread wear, with good wet and dry traction for the road and suitable stiffness to promote handling and steering performance for the tire.
A challenge is presented to promote a softer vehicular ride, translating to increased vehicular comfort in a sense of a softer vehicular ride in a manner which is a departure from past practice.
One way to promote a softer vehicular ride is to use a softer (reduced stiffness property) rubber composition for the tread.
The stiffness of a tread rubber composition can be made reduced, or made softer, for example, by use of a lower surface area particulate rubber reinforcing filler, by use of a reduced amount of reinforcing filler content in the rubber composition and/or by use of an increased amount of rubber processing oil content in the rubber composition.
However, such reduced stiffness (softer) tread rubber composition would be expected to lower desirable tire tread performance such as wet and dry road traction and reduced wear resistance which would lead to increased treadwear. Further, vehicular handling and/or steering would also be expected to be negatively affected.
For this invention, it is proposed to use an intermediate transition rubber layer underlying said tread cap rubber layer and between the tread cap rubber layer and a tread base rubber layer which is a softer rubber (reduced stiffness property) than the outer tread cap rubber.
A challenge is presented to provide such softer transition rubber layer (significantly softer than the tread cap rubber) composed of a significantly softer rubber composition (very low Shore A hardness) with a significantly high low strain modulus property in both the circumferential directional plane of the tire tread and the lateral directional plane of the tire tread to promote directional stability for the tire tread with a view toward promoting vehicular ride comfort as well as handling stability for the tread.
In this manner, desirable outer tread cap rubber wet and dry traction properties as well as treadwear resistance would intended to be maintained while a softer vehicular ride for the tire itself is promoted.
However, it is envisioned that use of the softer intermediate rubber layer would be expected to provide, or promote, reduced stiffness of the tire tread in its lateral directional plane (side-to-side direction), to thereby sacrifice handling and steering (turning) performance of the tire.
For the purposes of this invention, an evaluation of providing an inclusion of a dispersion of fibrillated aramid short fiber reinforcement within the softer rubber composition of the intermediate transition rubber layer its rubber composition to promote the stiffness of the intermediate tread rubber layer in both its circumferential and in its lateral direction (side-to-side direction), to thereby promote steering and handling performance of the tire.
It is appreciated that an orientation of the short fibrillated aramid fibers in the circumferential directional plane of the tread running surface, for the intermediate rubber layer, can be accomplished by conventional calendering or extrusion of the rubber composition to thereby promote reinforcement of the rubber composition layer in the circumferential directional plane of the tread intermediate layer.
However, it is an important aspect of this invention to evaluate whether such fiber orientation of the short fibrillated aramid fiber may also provide reinforcement of the rubber composition layer of the soft intermediate rubber layer of the tread in the lateral circumferential direction (side-to-side direction) of the tread intermediate layer.
In the description of this invention, the terms “rubber” and “elastomer” where used, are used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, where used, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing and rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.